Bock, Walter J. Department of Biological Sciences, Columbia University, New York, New York.
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A group of flightless, mostly large, running birds characterized by their flat, keelless sternum. Members of the ratites were formerly segregated as a superorder of birds, the Palaeognathae, but their interrelationships have been a long-standing controversy. Species include the ostriches (Fig. 1), emus, cassowaries, and rheas (Fig. 2). Traditionally, the flying tinamous, in spite of their also possessing the typical palaeognathous palate, were not included in this group, although being closely similar to the rheas. Ornithologists, stressing the disjunctive distribution of these flightless birds and the many possible convergent features due to their large size, have concluded that the several groups of ratite birds represent end points of unrelated phyletic lineages from several different ancestral avian stocks and hence should be placed in separate orders. Comparative morphologists, on the other hand, stressing the basic similarity of the bony palate and other anatomical features, have concluded that the ratites are a strictly monophyletic group of birds. The latter view is supported by evidence that the structure of the bony palate, the type of cranial kinesis, and other cranial features are homologous in all ratites, in spite of their other differences. Additional evidence such as general behavior, the structure of the rhamphotheca (the horny keratinous sheath of the beak), and some biochemical features also support this conclusion.
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